Drying method and apparatus



July 12, 1966 H. KLINKMUELLl-:R ETAL 3,259,994

DRYING METHOD AND APPARATUS Filed Dec. 20, 1962 2 Sheets-Sheet 1 S ,E AIUV July 12, 1966 Filed Deo. 20, 1962.

H. KLINKMUELLER ETAL 3,259,994

DRYING METHOD AND APPARATUS 2 Sheets-Sheet 2 5a x 60 t (mm) United States Patent O 3,259,994 DRYING METHOD AND APPARATUS Horst Klinkmueller and Friedrich Hirth, Stuttgart, Germany, assignors to Gann-Apparateund Maschinenbau G.m.b.H., Stuttgart S., Germany Filed Dec. 20, 1962, Ser. No. 246,104 Claims priority, applictign (ermany, Dec. 23, 1961, 3

The present invention refers to a method and apparatus for artificially drying solid material, particularly wood.

In the procedure of artificially drying more or less delicate or diflicult to treat natural or synthetic products as for instance wood (which is referred herebelow generally as being representative of the type of materials to 'be dried) care must be taken to avoid damage or deterioration of the material to be dried. Therefore it is necessary that the artificial drying procedure be controlled in -such a manner that it takes its course as far as possible in a manner similar to the conditions prevailing in a natural drying process with the excep-tion that the duration of the process is greatly reduced.

Therefore, in order to properly carry out an artificial drying process it is necessary to know and to take into consideration certain determining factors as for instance the mean moisture content of the material to be dried, the so-called equilibrium moisture content thereof and the drying gradient as well as the relations between these factors.

It is well known to determine the mean moisture content of solid material like wood by means of kiln-drying tests or by consecutively weighing `test pieces taken from the drying chamber at predetermined intervals during the drying operation. A better and more up-to-date known method consists in determining the mean moisture con- -tent of the material by measuring the electric resistivity of the material while the latter is in the drying chamber. Thus a continuous indication of the varying values of the mean moisture content is obtained in the form of an analogue electric value representing such mean moisture content.

The so-called equilibrium moisture content of a solid material is defined as the relative moisture content thereof vwhich the particular material displays after being exposed to a certain climate for an indefinite period until no change of moisture content takes place any more. The term climate is to be understood as the condition of the atmosphere surrounding the respective material and characterized by its temperature and relative humidity. Up t-o now the conventional method of determining the equilibrium moisture content of a material consisted in measuring the temperature and the relative humidity of the atmosphere surrounding the material and then to derive from the results of such measurements the applicable equilibrium moisture content value from tables, charts or the like which have been established empirically on the basis of experimentation and experience. Evidently this system of' determining the equilibrium moisture content of a material is cumbersome and time-consuming and almost useless if a drying procedure is intended to lbe carried out under automatic control.

In carrying out conventionally an artificial drying process the climate in the drying chamber is controlled either entirely by manual control or semi-automatically or in accordance with a precalculated program. Experience has shown that such a conventional procedure still requires continuous observation by well-trained and reliable personnel, yet very frequently serious mistakes occur which are mainly due to the human factor involved in determining the equilibrium moisture content existing at a given moment. On the other hand, in conventional semi-automatic or fully automatic drying procedures the danger always exists that the decrease of moisture content of the material to be dried, particularly of wood, does not tally with the precalculated program so that a need exists for certain precautions to be taken which, however, necessarily entail a great delay in the operation and thus an increase of the duration of the entire drying procedure.

The above mentioned drying gradient is a factor a defined by the equation wherein um is the actually existing mean moisture content of the material to be dried, while um is the equilibrium moisture content of the particular material which is intended to be achieved at the end of the drying operation or of a predetermined portion thereof. It is one object of this invention to provide for a method by which the existing equilibrium moisture content of a material to be dried can be determined exactly and practically without delay because only yby doing this it is possi-ble to control the drying process correctly particularly if the drying process is to be completed within a period of time substantially shorter than the time required for obtaining the same result by means of naturally drying the material.

It is another object of this invention to provide for a drying method and apparatus whereby it is possible to keep during the drying operation as constant as possible the drying gradient a which is a factor based on experimentation or experience and having a different value for instance for different types of wood and sizes of the pieces of wood to be dried.

It is still another object of the invention to provide for a method and apparatus permitting to measure continuously during an artificial drying operation the varying equilibrium moisture content -of the material to be dried las well as its varying means moisture content, all the difficulties and weaknesses of the above mentioned conven tional method 'being practically completely eliminated.

With above objects in view the invention includes a method of determining the equilibrium moisture content of a solid material, comprising the steps of: exposing a thin-Walled specimen of a solid material capable of absorbing humidity to an atmosphere having a given humidity and a given temperature, said specimen having a surface/thickness ratio permitting it to display a varying equilibrium moisture content corresponding to variations of the humidity and temperature characteristics of said atmosphere; and measuring the moisture content of said specimen whereby at a given moment the equilibrium moisture content of said material relative to the characteristics of said atmosphere substantially at said given moment is represented by said measured moisture content of said specimen.

In another aspect of the invention it includes a method of artificially drying solid material, particularly wood, comprising the steps of: exposing solid material to be dried to a variable climate; measuring the mean moisture content of said solid material during its exposure to said variable climate, said mean moisture content varying as a result of said exposure to said variable climate; determining in said variable climate the equilibrium moisture content characteristic of said solid 1material when exposed to said variable climate; determining any deviation of said mean moisture content from a reference value equal to a times the value of said determined equilibrium moisture content wherein the factor a is a desired drying gradient; and regulating said variable climate in dependency upon said thus determined deviation in a manner tending to eliminate such existing deviation.

-In still another aspect the invention includes an arrangement for controlling a process of artificially drying a solid azllm/llGl material, particularly wood, in a drying chamber containing an atmosphere of variable climate and equipped with control means for varying said climate, comprising, in combination, first electric circuit means adapted to measure the variable mean moisture content of the solid material in the drying chamber by measuring the resistivity of said material and furnishing a first electrical analogue value representing said mean moisture content of said material; second electric circuit means adapted to measure the variable equilibrium moisture content of a specimen of said solid material in the drying chamber'by measuring the resistivity of said specimen and furnishing a second electric analogue value representing said equilibrium moisture content of said specimen; electric conversion means for converting said first analogue value into a reference value by reducing said first analogue value in the proportion um/ a wherein um is Said first analogue value representing said mean moisture content and a is a factor representi-ng a desired drying gradient; and comparator means connected with said second circuit means and with said conversion means for converting any existing difference between said second analogue value and said reference value into a control signal adapted to be applied to the control means of the drying chamber for changing the prevailing climate therein in a manner tending to eliminate said difference.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and itsy method of operation, together with additional objects and advantages hereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram illustrating an arrangement according to the invention and simultaneously illustrating the method according to the invention; and

FIG. 2 is a diagram illustrating the variation of certain factors during the execution of the method according to the invention.

Before starting a description of the method and arrangement according to the invention in detail, it is to be stated that the method and arrangement described hereafter are based on the premise that the mean moisture content of wood or other material to be dried existing at a given moment is the lead factor. The equilibrium moisture content of said material constitutes a value which is to be regulated and it constitutes a reference value defined by the equation wherein um is the mean moisture content of the material measured at a given moment while a is a factor representing a desired drying gradient. Thus the reference value HG1 is the value which is desired to be adjusted to assume a desired value, in contrast to the equilibrium moisture content actually Imeasured at said given moment.

In FIG. 1 only those components of an arrangement according to the invention are shown which are necessary to understand it while the actual drying chamber and the means for controlling the climate therein are omitted since they do not form part of this invention.

FIG. 1 shows electric circuit means adapted to measure the variable mean moisture content um of one or more pieces of solid -material like wood as indicated at W1, W2 and W3 by measuring the resistivity of that material. For this purpose a voltage of e.g. 30 volts is applied to a measuring circuit comprising two electrodes attached to each piece of material (of known length and cross section) and a measuring amplifier 1, 2, 3, respectively, as is well kgwn in the art. Each of the above mentioned circuits or more particularly amplifiers is adapted to furnish an electrical analogue value eg. a voltage representing the measured -mean moisture content um of the representative piece of'material W1, W2, W3. The just mentioned electrical analogue value is furnished to an indicator 4 which may be a contact voltmeter of conventional type. Evidently, this device could as well be an ammeter or contactammeter.

It will be understood that the arrangement could also be used with only one of the above mentioned measuring circuits i.e. only with one amplifier 1 furnishing an analogue value corresponding to the resistivity of only the piece W1. However, it is preferred to use a plurality of these circuits and to connect them with the device 4 so that a vmean value of resistivity of a plurality of pieces like W1 to W3 is formed and indicated by the indicator 4 and furnished thereby to the remainder of the circuit arrangement.

The indicator 4 which also constitutes the output means of the above described measuring circuit means is connected with electric conversion means for converting the above mentioned first analogue value into a reference value by reducing the analogue value representing the mean moisture content of the solid material in the drying chamber in the proportion 14m/a i.e. by dividing the just mentioned analogue value by the above explained factor rz which represents a desired drying gradient. A suitable form of the electric conversion means is a voltage or current divider 5 which is adjustable for the above mentioned purpose and is connected at one end with one of the inputs A of a comparator device 7 described further below while its terminal carrying the variable voltage or current is connected to the second one of the inputs A of the comparator 7 via a change-over switch 6 described further below. Of course, in case the change-over switch 6 is omitted the last mentioned connection may be a direct one.

The arrangement according to FIG. l comprises also second circuit means for measuring the equilibrium moisture content HG1 of a specimen W4 of the solid material mentioned above. The specimen W4 is also located in the drying chamber so as to be exposed to the same climate, i.e., humidity and temperature as the pieces W1 to W3. The specimen W4 is a very thin slice of the above mentioned solid material having a surface versus thickness ratio which permits it to assume upon exposure to a given climate the corresponding equilibrium moisture content characteristic of the particular material practically without delay so that the resistivity of this specimen constitutes a second electrical analogue value representing said equilibrium moisture content. For example in the case of the solid material being pine wood the specimen may have suitably a thickness of 1 mm., a Width of 50 mm. and a length of 20 mm. In the same manner as mentioned above the resistivity of the specimen may be measured by attaching to it two electrodes spaced from each other a predetermined distance land connected in a circuit supplied., e.g., with /5 volts and containing an amplifier 8 furnishing a voltage or current constituting that second electrical analogue value representing the moisture content or, under the given conditions, the equilibrium moisture content of the specimen W4 to the indicator 9. From this indicator the voltage or current constituting said second electrical analogue value is supplied to the inputs B of the comparator 7.

It will -be understood that the comparator device 7 of conventional type is supplied, on one hand, at its inputs A with a reference value` equal to the first electrical analogue value divided by the factor a, .i.e., proportional to the fraction um/a, while the comparator 7 is supplied -at its other inputs B with the second electrical analogue value representing the equilibrium moisture content um of the speci-men W4. As long or if the comparison of said reference value with said second electrical analogue value results in a difference therebetween a control signal of corresponding magnitude or characteristic is supplied from the output C of the comparator 7 to the control means of the drying chamber for changing the prevailing climate therein in a manner tending to eliminate said difference` In this manner the climate in the drying chamber is controlled automatically in the desired manner.

Preferably the measurement of the mean moisture content um of the solid material by the tirst mentioned circuit means as well as the measurement of the equilibrium moisture content uGl of the specimen W4 is carried out continuously and simultaneously. However, it should be understood that continuously also includes periodically.

It is desirable to automatically terminate the drying process when -the mean moisture content um of the solid material has been reduced to a certain value. When this point has been reached it is further desirable to further condition the materi-al in a climate, somewhat different from the one prevailing at the end of the actual drying operation. For this purpose the arrangement illustrated in FIG. l contains means for providing a second reference value representing an adjustable value of equilibrium moisture content, said second reference value being applied to the inputs A of the comparator 7 during the conditioning period instead of the previouslyused, reference value furnished by the conversion means described above. The means for applying said second reference value may consist in a voltage or current divider 10 supplied with electric energy from an outside source as indicated and connected at one terminal thereof with one of the inputs A ofthe comparator 7 while the terminal of the divider 10 carrying an adjusted voltage or current is connected with that terminal of the change-over switch 6 which is normally not engaged by the movable contact arm of said switch. The switch 6 may be of the relay type or otherwise controllable by an actuating signal for being moved from its normal position as illustrated to its second position. In this case the indicator device 4 may be provided with a rotary member 4 adapted to close in a predetermined position a contact pair 4 the closing of which would result in issuing the above mentioned actuating signal to the switch 6. The .predetermined position in which the Contact pair 4 is closed is selectable or adjustable so as to correspond to a selected final value of the mean moisture content um of the solid material in the drying chamber.

The above description of the arrangement contains implicitly also a description of the method that can be carried out for instance with the aid of the described arrangement. FIG. 2 illustrates the method according to the invention. In the diagram of FIG. 2 the ordinates indicate the mean moisture content um and the equilibrium moisture content um in percentage of moisture in the material while the abscissae indicate time. In the lefthand portion of the diagram the line marked um indicates the variation of the mean moisture content during a given period of time while the line um indicates the concurrently appearing changes in the equilibrium moisture content as measured at the specimen W4. As can be seen from this example the original value of um amounting to 39% drops to about 11.5% while simultaneously in cornpliance with the Equation 2 the value um drops from 13% to about 4%. As soon as the end of the actual drying process is reached, i.e., when um reaches 11.5 the a-bove mentioned switch 6 would be moved to its second position provided that the indicator 4 was so set as to cause closing of the contact pair 4 when um reaches 11.5%. Consequently after the change of the switch 6 to its second position the climate in the drying chamber will be further controlled so that the equilibrium moisture content measured at the specimen W4 rises to the value determined by the second reference value, i.e., the setting of the device 10 as can be seen in the right-hand half of the diagram of FIG. 2. This portion of the diagram represents the conditioning period and it can be seen that the value um approaches asymptotically the adjusted value um near the start of the conditioning period.

j It will be understood that each of the elements described above, or two or more together, may also nd a useful application in other types of drying method and apparatus for solid materials diiering from the types described above.

In a practical example of the apparatus there are used microammeters pa.) as indicators 4 and 9 and a microammeterv with two separate and opposing measuring coils as comparator 7.

While the invention has been illustrated and described as embodied in automatic drying method and apparatus for solid materials, it is not intended to be limited to the details shown, since various modiiications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily ladapt it for various applications without omitting features tha-t, yfrom the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this inventiomand, therefore, such adaptations should and are intended to be comprehended within the meaning and range ofequivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is: v

1. In a me-thod of drying solid material, particularly wood, the steps of: exposing solid material to be dried in a drying chamber to a variable climate in said drying chamber; measuring Ithe mean moisture content of said solid material during its exposure to said'variable climate, said mean moisture content varying as a result vof said exposure to said variable climate; determining in said variable climate the equilibrium moisture content characteristic of said solid material when exposed to said Variable clima-te by measuringvthe moisture content of a thin specimen of said material exposed to said climate, said specimen having a surface/thickness ratio permitting it to display an equilibrium moisture content varying in conformity with variationsV of said variable climate; determining any deviation of said mean moisture content from .a reference Value equal t0 a times the value of said determined equilibrium moisture content wherein the factor a is a desired drying gradient; and regulating said variable climate in said drying chamber during drying said solid natural therein in dependency upon said this determined deviation in a manner tending -to eliminate such existing deviation.

2. In |a method of drying solid material, particularly wood, the steps of: exposing solid material to be dried in a drying chamber to a variable climate in said drying chamber; continuously measuring the mean moisture content of said solid material during its exposure to said variable climate, said mean moisture content varyas a result of said exposure to said variable climate; continuously determining in said variable climate the equilibrium moisture content characteristic of said solid material when exposed to said variable climate =by measuring the moisture content of a thin specimen of said material exposed to said climate, said specimen having a surface/thickness ratio permitting it to display an equilibrium moisture content varying in conformity with variations of said variable climate; continuously determining any deviation of said mean moisture content from a reference value equal to a times the value of said determined equilibrium moisture content wherein the factor a is a desired drying gradient; and regulating said variable climate in said drying Chamber during drying said solid material therein in dependency upon said this determined deviation in a manner tending to eliminate such existing deviation.

3. A method of articially drying solid material, particularly wood, comprising the steps of: exposing the solid material to be dried in la drying chamber to a variable climate existing in said drying chamber and characterized by the humidity and temperature of the atmosphere therein continuously; measuring the mean moisture content of said material in said chamber during its exposure to said climate, said mean moisture content varying as a result ofV said exposure; deriving from the measured varying values of said mean moisture content of said solid ma-terial a reference value by dividing said measured values of said mean moisture content by a factor representing a desired drying gradient a=um/L4G1 wherein um is the mean moisture content an-d HG1 is the equilibrium moisture content; simultaneously exposing a thin specimen of said material to said variable climate in said drying chamber, said specimen having a surface/thickness ratio permitting it to display a varying equilibrium moisture content in conformity with the variable climate in said drying chamber existing :at a given moment continuously; measuring said varying equilibrium moisture content of said specimen at said given moment simultaneously with measuring said mean moisture content of said solid material; comparing said reference value with the measured value of said equilibrium moisture content of said specimen so as to obtain a difference value characteristic of an existing difference between an existing climate in said'drying chamber and a desired climate in said drying chamber during drying said solid material therein; and regulating said climate in a manner tending to eliminate an existing difference between the existing and the desired climate.

4. An arrangement for controlling a process of artificially drying a solid material, particularly wood, in a drying chamber containing an atmosphere of variable climate and equipped with control means for varying said climate, comprising, in combination, first electric circuit means adapted to measure the variable mean moisture content of the solid material inthe drying chamber by measuring the resistivity of said material and furnishing a first electrical analogue value representing said mean moisture content of `said material; a specimen of the material to be dried located in said drying chamber and subjected to said variable climate, said specimen having a surface/thickness ratio so as to assume substantially without delay the equilibrium moisture content corresponding at any moment to said variable climate in said drying chamber; second electric circuit means connected to said specimen to measure the variable equilibrium moisture content thereof in the drying chamber by measuring the resistivity of said specimen and yfurnishing a second electric analogue Value representing said equilibrium moisture content of said specimen; electric conversion means for converting said rst analogue value into a reference value by reducing said rst analogue value in the proportion 14m/a wherein um is said first analogue value representing said mean moisture content and a is a factor representing a desired drying gradient; and comparator means connected with said second circuit means and with said conversion means for converting any existing difference between said second analogue value and said reference value into a control signal adapted to be applied to the control means of the drying chamber for changing the prevailing climate therein in a manner tending to eliminate said difference.

`5. An arrangement for controlling a process of artificially drying a solid material, particu-larly iwood, in a drying chamber containing an atmosphere of variable climate and equipped with control means for varying said climate, comprising, in combination, first electric circuit means adapted to measure the variable mean moisture content of the solid material in the drying chamber by measuring the resistivity ofv said material and including amplifier means exposed to substantially the same temperature as said solid material in said drying chamber furnishing a 'first electrical analogue yvalue representing said mean moisture content of said material; va specimen of the material to be driedv located in said drying -chamber and subjected to said Vvariable climate, said specimen having a surface/thickness ratio so as to assume substantially without delay the equilibrium moisture content corresponding at any moment to said variable climate in Said drying chamber; second electric circuit means connected Ito said specimen to measure kthe variable equilibrium moisture content thereof in the drying `chamber by measuring the resistivity of said specimen and including amplier means exposed to substantially the same temperature as said solid materia-l in said drying chamber furnishing a second electric analogue value representing said equilibrium moisture con-tent of said specimen; electric conversion means for converting said first analogue value into a reference value by reducing said first analogue value in the proportion um/a wherein um is said first analogue value representing said mean moisture content and a is a factor representing a desired drying gradient; and comparator means connected with said second circuit means and with said conversion means for converting any existing difference between said second analogue value and said reference value into a control lsignal adapted to be applied to the control means of the drying chamber for changing the prevailing climate therein in a manner tending lto eliminate said difference.

46. An arrangement for controlling a process of artificially drying a solid material, particularly Wood, in a drying cham-ber -containing an atmosphere of variable climate and equipped with control means yfor varyin-g said climate, comprising, in combination, first electric circuit means adapted to measure the variable mean mois-ture content of the sol-id material in the drying chamber by measuring the resistivity of said material and including amplifier means exposed to substantially the same temperature as said solid material in said drying chamber furnishing a first electrical analogue value representing said mean moisture content of said material, said amplifier mean-s including temperature-compensating means comprising a temperature-responsive resisto-r means having a temperature coefiicient substantially equal to and opposite to that of said solid material so that said first electrical analogue value is linearly proportional to said mean moisture content of said material at least within the temperature range occurring in said drying chamber; a specimen of the material to be dried located in said drying chamber and subjected to said variable climate, said specimen having a surface/ thickness ratio so as to assume substantially without delay the equilibrium moisture content corresponding at any moment to said variable climate in said drying chamber; second electric circuit mean-s connected to said specimen to measure the Variable equilibrium moisture content thereof in the Idrying chamber by measuring the resistivity of said specimen and including amplifier means exposed to substantially the same temperature as said solid material in said drying chamber and furnishing a second electric analogue value representing said equilibrium moist-ure content lof said specimen, Said amplifier means including temperature-compensating means comprising a temperature-responsive resistor means having a temperature coefficient substantially equal to and opposite to that of said specimen of said solid material so that said second electrical analogue value is linearly proportional to said moisture content of said specimen of said material at least within the temperature range `occurring in said drying chamber; electric conversion means for converting said iirst analogue value into a reference value by reducing said Itirst analogue value in the proportion 11m/a wherein um is said Ifirst analogue value representing said mean moist-ure content and a is a factor representing a desired drying gradient; and comparator means connected with said second circuit means yand with said conversion means for converting any existing difference between said second analogue value and said reference value into a control signal adapted to be applied to `the control means of the drying chamber for changing the prevailing climate therein in a manner tending to eliminate said difference.

7. An arrangement as claimed in claim 4, including electrical control mean-s )for applying a second electrical reference Value to said comparator means, said second reference value representing a desired equilibrium moisture content, and change-over switch means connecting in .a rst position said conversion means with said comparator means and connecting in a second position said electrica-l control means with said comparator means, said changeover switch means being normally in said iirst position `and being movable to said second position by application thereto of an actuating signal, said rst circuit means including means for issuing said actuating signal 'when said iirst electrical analogue value representing said mean moisture content reaches a predetermined value, so that thereafter said comparator means will compare -said second analogue value with said second reference value and issue in the case of an existing difference therebetween a control signal adapted to be applied to the control means of said drying chamber for .changing the prevailing climate therein in a manner tending to eliminate said 'last mentioned difference.

8. An arrangement as claimed in claim 5, including 'electrical control means `for applying a second electrical reference value -to said comparator means, said second reference value representing a desired equilibrium moisture content, and change-over switch means connecting in a ii-rst position said conversion means with said cornparator means and connecting in a second position said electrical control means with said comparator means, said change-over switch means being normally in said lirst position and being movable -to said second position 'by application thereto of an actuating signal, said tirst circuit means including means for issuing said actuating signal when said first electrical analogue value representing said mean moisture content reaches a predetermined value, so that thereafter said comparator means will compare said second analogue value with said second reference value and 4issue in the case of an existing difference therebetween -a control signal adapted to be applied to the control means of said drying chamber for changing the prevailing climate therein in a manner tending to eliminate said last mentioned difference.

9. An arrangement Ias claimed in claim 6, including electrical control means for applying a second electrical reference value to said comparator means, said second reference value representing a desired equilibrium moisture content, and change-over switch means connecting in a lirst position said conversion means with said cornparator means and connecting in a second position said electrical control means with said comparator means, said change-over switch mean-s being normally in said iirst position and being movable to said second position by application thereto oif an actuating signal, said rst circuit means including means rfor issuing said actuating signal when said first electrical analogue value representing said mean moisture content reaches a predetermined value, so that thereafter said comparator means will compare said second analogue value with said second reference Value and issue in the case of an existing difference therebetween -a control signal adapted to be applied to the control means of said drying chamber for changing the prevailing climate therein in `a manner tending to eliminate said last mentioned diieren-ce.

References Cited by the Examiner UNITED STATES PATENTS 2,288,154 6/1942 Cobb 34-26 2,448,144 8/ 1948 Guthier 34-46 2,484,527 10/ 1949 Rhoads et al. 34-46 2,763,069 9/ 1956 Vaughan 34-26 2,906,034 9/1959 Robinson et al. 34-26 3,070,896 1/1963 Knudson et al. l34-26 3,131,034 4/1964 Marsh 34-30 WILLIAM F. ODEA Primary Examiner.

NORMAN YUDKOFF, Examiner.

F. E. DRU-MMOND, C. R. REMKE,

Assistant Examiners. 

1. IN A METHOD OF DRYING SOLID MATERIAL, PARTICULARLY WOOD, THE STEPS OF: EXPOSING SOLID MATERIAL TO BE DRIED IN A DRYING CHAMBER TO A VARIABLE CLIMATE IN SAID DRYING CHAMBER; MEASURING THE MEAN MOISTURE CONTENT OF SAID SOLID MATERIAL DURING ITS EXPOSURE TO SAID VARIABLE CLIMATE, SAID MEAN MOISTURE CONTENT VARYING AS A RESULT OF SAID EXPOSURE TO SAID VARIABLE CLIMATE; DETERMINING IN SAID VARIABLE CLIMATE THE EQUILIBRIUM MOISTURE CONTENT CHARACTERISTIC OF SAID SOLID MATERIAL WHEN EXPOSED TO SAID VARIABLE CLIMATE BY MEASURING THE MOISTURE CONTENT OF A THIN SPECIMEN OF SAID MATERIAL EXPOSED TO SAID CLIMATE, SAID SPECIMEN HAVING A SURFACE/THICKNESS RATIO PERMITTING IT TO DISPLAY AN EQUILIBRIUM MOISTURE CONTENT VARYING IN CONFORMITY WITH VARIATIONS OF SAID VARIABLE CLIMATE; DETERMINING ANY DEVIATION OF SAID MEANS MOISTURE CONTENT FROM A REFERENCE VALUE EQUAL TO A TIMES THE VALUE OF SAID DETERMINED EQUILIBRIUM MOISTURE CONTENT WHEREIN THE FACTOR A IS A DESIRED DRYING GRADIENT; AND REGULATING SAID VARIABLE CLIMATE IN SAID DRYING CHAMBER DURING DRYING SAID SOLID NATURAL THEREIN IN DEPENDENCY UPON SAID THIS DETERMINED DEVIATION IN A MANNER TENDING TO ELIMINATE SUCH EXISTING DEVIATION. 